Conventionally, painting is often employed as a method for decorating molded products molded with thermoplastic resins.
According to a typical painting method, a molded product formed by injection molding is removed from the mold, is coated with paint by, for example, spraying or dipping, and is dried to form a hard coating covering the surface of the molded product for decoration and protection. The term “coating material” may hereinafter be used as a synonym of “paint”.
In recent years, however, in-mold coating (which may be referred to as in-mold coating) has been proposed, in which resin is molded and coated in the same mold to eliminate the step of painting.
FIG. 12 shows a flow chart of an example of a known in-mold coating method. According to the in-mold coating method in FIG. 12, a thermoplastic resin, namely a base material, is molded by injection molding and is cooled to some extent. The mold is then slightly opened to form a gap between the resin molded product formed in the mold and the mold cavity. Paint is injected into the gap with a paint-injector. The mold is clamped again to uniformly spread the paint over the surface of the molded product. The spread paint is cureed to form a coating.
Because the thermoplastic resin is molded and coated in the same mold, the in-mold coating method involves fewer steps to achieve cost reduction. In addition, this method poses almost no problems such as defects caused by airborne dust adhering to a wet coating to provide high-quality products.
Studies have been made on the use of the in-mold coating method particularly for many automobile parts, such as bumpers, doors, door mirror covers, and fenders, which need high quality in external appearance.
Examples of the in-mold coating method are disclosed in, for example, Japanese Unexamined Patent Application Publication Nos. 11-277577 (Patent Document 1), 2000-141407 (Patent Document 2), 2000-334800 (Patent Document 3), and 2001-38737 (Patent Document 4).
According to the above known in-mold coating methods, however, mold-clamping force cannot be exerted on paint, namely a coating material, in reclamping if the amount of paint injected is small. Unfortunately, therefore, these methods cannot form a uniform coating on the overall coating surface of a resin molded product.
One of the causes of the nonuniform coating thickness is the decrease in the volume of the molded resin due to thermal contraction in the mold. When the thickness of the molded product decreases by thermal contraction, a gap occurs between the resin molded product and the cavity surface in the cavity. The gap is difficult to fill with paint in an amount equivalent to the desired thickness. As a result, the paint cannot be spread over the surface to be coated, and a uniform coating cannot be formed.
To solve the above problem, the amount of coating material injected may be increased to solve the problem of defective transfer of the mold surface to the coating material, though this approach poses another problem of a larger coating thickness than necessary.
Another cause of the nonuniform coating thickness is mold deformation in molding. A typical mold has high rigidity, but deforms by several to tens of micrometers when clamped with a clamping machine. Such degrees of deformation are often negligible in general resin molding. In in-mold coating, however, a resin molded product is generally coated with a coating having a thickness of about tens of micrometers. Thus the deformation of the mold cavity due to mold deformation is listed as one of the causes of a nonuniform coating thickness.
According to a known in-mold coating method, particularly, mold-clamping force after paint injecting may be changed stepwise. This method, however, results in a nonuniform coating thickness because the degree of mold deformation varies depending on the magnitude of the mold-clamping force to cause variations in the distance between the surface of a resin molded product and the mold cavity.
Referring to FIGS. 10(a) to 10(d), a gap may be formed between a resin molded product formed under general injection molding conditions and a mold cavity surface by thermal contraction of molten resin charged into the mold cavity even if the mold is not opened. Because the size of the gap varies depending on, for example, the shape of the mold cavity and the thickness of the resin molded product, a uniform gap rarely occurs over the surface to be coated. In in-mold coating, therefore, the mold must be opened to ensure a necessary coating thickness. If, however, the mold is opened, the resultant coating is undesirably extremely thick at a portion where the gap formed by thermal contraction combines with that formed by opening the mold.
If molten resin with a volume exceeding the capacity of the mold cavity is charged into the mold cavity by applying excessive charging pressure to compensate for the volume of thermal contraction of the resin, the excessive charging pressure undesirably causes, for example, variations in the thickness of a resin molded product, as shown in FIGS. 11(a) to 11(d). In an embodiment of the related art, in FIGS. 11(a) to 11(d), the excessive injection pressure causes a difference in thickness between the ends of the resin molded product and the portion around a gate part.
In addition, excessive reclamping pressure after paint injecting is undesirable because the pressure causes a phenomenon called a hump at a thick portion such as a rib and a boss. To prevent the defect, mold-clamping force after the injecting of coating material must be reduced, though a large difference between mold-clamping force in resin injection and that after the injecting of coating material undesirably results in a nonuniform coating thickness.